The myeloproliferative neoplasms (MPNs) are clonal stem cell disorders characterized by stem cell expansion and overproduction of mature blood cells. The acquired kinase mutation JAK2V617F plays a central role in these disorders, but the precise molecular mechanisms responsible for MPN stem cell expansion are not fully understood, limiting the effectiveness of current treatments. Abnormalities of the hematopoietic microenvironment (niche) are beginning to be recognized as an important factor in the development of hematologic malignancies including MPNs. Endothelial cells (ECs) are an essential component of the hematopoietic niche and most hematopoietic stem cells reside adjacent to a marrow sinusoid (the ?vascular niche?). Patients with MPNs are characterized by increased marrow angiogenesis compared to normal marrow and ECs carrying the JAK2V617F mutation can be detected in these patients. Our recent work demonstrated that the JAK2V617F-bearing vascular niche not only promotes the expansion of JAK2V617F-mutant stem cells in preference to JAK2WT stem cells but also protects the mutant cells from lethal irradiation administered during conditioning for marrow transplantation. The levels of CXCL12, an essential niche factor for stem cell maintenance and survival, are increased in JAK2V617F-bearing ECs compared to JAK2WT ECs. In addition, we found that thrombopoietin (TPO) and its receptor MPL, two key regulators of stem cell activity, are also important for the vascular niche function and MPL is essential for MPN stem cell expansion and the development of myeloproliferative phenotype in the JAK2V617F-bearing vascular niche. The objective of the proposed work is to determine the physiological effects and the molecular mechanisms by which the JAK2V617F mutation alters the hematopoietic vascular niche to promote MPN stem cell expansion. In particular, we propose the following two specific aims: Aim 1) To test the hypothesis that the JAK2V617F mutation alters vascular niche function to promote MPN stem cell expansion and disease relapse after stem cell transplantation. The roles of CXCL12 in JAK2V617F-bearing vascular niche function in MPNs will be determined. In addition, the effects of the JAK2V617F mutation on human vascular endothelium function will be assessed. Aim 2) To test the hypothesis that the JAK2V617F mutation changes vascular niche function in MPNs via altered TPO/MPL signaling. The long term goal of this research proposal is to define the molecular and cellular functions of the hematopoietic vascular niche in both normal and neoplastic hematopoiesis, and to develop more effective therapeutic strategies for patients with MPNs and potentially other hematologic malignancies.